Windows messenger cannot see online contacts

I run XP home on my netbook and I have downloaded windows live messenger. I can send and receive emails as I did try with my other non hotmail address, but my contacts in messenger always shows as offline when I know that they are not, they cannot contact me neither.
What's wrong with this? Why can't I see them online when they are?

Thanks, in view of the thread you have directed me to, I had decided to uninstall the messenger version I had and download it again.
Going through "add or remove" in control panel, it gave me the option to repair windows live essentials, so I tried to repair it before removing it, and it seems to work.